1. Technical Field
This invention relates to a method of dyeing a thermoplastic resin article, especially a thermoplastic resin plastic lens, and to a colored plastic lens obtainable by that method. More specifically, the invention relates to a method of dyeing a thermoplastic resin article which can effectively dye the article at a low temperature within a short period of time, and to a colored plastic lens obtained by that method.
2. Description of the Related Art
Cast polymerization or injection molding is usually employed for molding plastic lenses, especially spectacle lenses. A polycarbonate comprising bisphenol A is known as a raw material for molding lenses by injection molding. When a polycarbonate comprising bisphenol A is used as the raw material, the manufacturing time of lenses can be shortened, and good impact resistance can be imparted. Accordingly, a number of thermoplastic lenses obtained by injection molding have been proposed.
Generally employed methods of dyeing plastic lenses include dyeing with one or more disperse dyes using water as a dispersion medium. Further, in the case of plastic lenses that are difficult to dye with disperse dyes using water as the dispersion medium, it has been proposed to use carrier dyeing, in which trichlorobenzene or the like is added to the dyeing liquid, pressure dyeing, a method in which an organic dye is heated and sublimated in a vapor phase to achieve dyeing, and hardcoat film dyeing.
However, thermoplastic resins such as polycarbonates comprising bisphenol A are highly hydrophobic. Therefore, it is difficult to perform dyeing with a disperse dye, particularly high-density dyeing, with these usual dyeing techniques.
Where the above-described various conventional dyeing methods are employed for making thermoplastic resin plastic lenses, the art encountered the following problems.
In dyeing with a disperse dye using water as a dispersion medium, it is necessary to perform the dyeing at a relatively high temperature. Accordingly, it is difficult to apply this dyeing technique to thermoplastic resin plastic lenses having a low glass transition temperature.
In carrier dyeing, thermoplastic resins are inferior to thermosetting resins in resistance to solvents. Accordingly, the carrier materials generally used in optical plastic lenses can invade the lens surface so that the necessary degree of transparency may be lost. Further, conventional carriers can contain, as a major component, trichlorobenzene, dichlorobenzene, phenylphenol, diphenyl, methylnaphthalene, or the like. These compounds are considered to be potentially damaging to the environment or to adversely affect human health. Thus, when these environmental and health problems are taken into consideration, carrier materials containing such compounds as major components are considered undesirable for future use.
The pressure applied in pressure dyeing may damage or destroy the optical performance qualities of plastic lenses.
The method in which an organic dye is heated and sublimated in a vapor phase to achieve dyeing is difficult to apply to thermoplastic resins, since such resins are generally inferior in heat resistance.
In hardcoat film dyeing, since it is difficult to put a large amount of a dye in a hard film, it is difficult to achieve high-density dyeing. In the case where the film thickness is made thick in order to contain a large amount of dye, problems such as crack generation in the hardcoat film may distort the lens shape, and film hardness may become low Moreover, the light resistance provided by hardcoat dyeing is poor as compared with methods in which the dye is directly applied to or incorporated in the thermoplastic resin substrate.
In the method in which a dye is dissolved in the raw material (monomer or monomer mixture) of the plastic lens substrate and then polymerized, the coloring density of the formed lens depends upon the thickness of the lens. For example, in concave lenses in which the central portion of the lens is thinner than the surrounding thereof, the color of the central portion of a lens made by this method is paler than the color of the surrounding portions. For this reason, there is a possibility that the color density varies considerably over the whole of the lens. Also, in lenses for spectacles having a different diopter from each other, there is a possibility that the color density is different between the left and right lenses. It is practically impossible to manufacture colored optical plastic lenses of a number of varieties while preparing a number of raw material liquids having varied color tones and densities depending upon the market needs, so this problem is particularly troublesome.
In addition, methods in which compounds having a number of dyeing sites are added to a substrate for the purpose of improving dyeability cannot be put into practical use because when these compounds are compounded in the substrate, the abrasion resistance and transparency of the substrate are lowered.